Wanting to lose 42 pounds!

I'm completely new to the whole fitness scene, but for years I've wanted to get into seriously working out and getting healthy. I've always envied other women who can work out constantly and keep their motivation and eat right, too. Well, the past week I've decided that it's time for me to get in shape. I currently weigh 162, I'm 5 ft 2 and I'd love to weigh 120 again, so I need to lose around 42 pounds. In my teen years, I've gained a ton of weight and since I'm turning 20 next month, I've decided that I'd like to do something good for myself and be healthy again! it sucks to let yourself go, but I plan to get back to where I'd like to be.